Glasses that are controllably crystallizable by a heat treatment are commonly referred to as thermally crystallizable glass compositions. The glass-ceramics are the products obtained from these controllably crystallizable inorganic glasses by a suitable heat treatment, and glass-ceramics are also referred to as thermally crystallized glasses. Thus the term "noncrystalline glass" excludes glass-ceramics but for convenience the term "glass" is used in this application to provide such exclusion and, therefore, to mean noncrystalline glass.
There are many types of silicate glasses that are thermally crystallizable glass compositions. A glass-ceramic body contains many small crystals in a glass matrix. The crystalline phase of glass-ceramics can contain one or more crystalline materials. The crystalline materials that are formed depend upon the original composition of the thermally crystallizable glass and often depend upon the nature of the heat treatment.
The expansion coefficient of thermally crystallizable glass is dependent upon the glass composition. There can be a substantial difference between the expansion coefficients of thermally crystallizable glasses that are not members of the same glass system. Also, the expansion coefficients of the glass-ceramics can differ greatly. The actual expansion coefficient of a glass-ceramic depends on the compositional ingredients and on the temperatures and times of the heat treatment used to form the glass-ceramic from the thermally crystallizable glass.
Articles of glass-ceramic material are made by melting batch ingredients to provide molten thermally crystallizable glass and thereafter forming from the molten glass by conventional means, such as press molding, casting, blow molding, and tube and rod drawing, useful glass articles. One type of useful article is tableware such as plates, cups, and tea pots. Tableware is usually made by pressing in a mold or by blow molding techniques. The articles of thermally crystallizable glass are subjected to a controlled heat treatment to convert the glass to a glass-ceramic.
Some glass-ceramics are compositions that contain one or more alkali metals, expressed as oxide as part of an overall composition also expressed primarily as oxides. Many of the thermally crystallizable glass compositions are of the lithia-alumina-silica system containing a minor amount of at least one nucleating agent for the glass, such as ZrO.sub.2, TiO.sub.2 and SnO.sub.2. By controlled in situ crystallization there is obtained glass-ceramic that contains in a glass matrix predominantly lithia-containing crystalline phases, either beta-eucryptite or beta-eucryptite-like crystals or beta-spodumene or beta-spodumene-like crystals, or both, as indicated by X-ray diffraction data.
Copending U.S. patent application Ser. No. 352,958, filed on Mar. 18, 1964 now U.S. Pat. No. 3,380,818, by William E. Smith and entitled "Glasses, Ceramics and Method", with a common assignee, discloses and claims another class of glasses and glass-ceramics that comprise silica, alumina, lithia, magnesia and a limited amount of both zirconia and titania. That application is hereby incorporated by reference.
U.S. application Ser. No. 352,958 describe s the manner of heat treatment to convert the crystallizable glass composition to glass-ceramic. The maximum temperature reached in the heat treatment ranges from about 1400.degree. to 2100.degree. F. and the period of time at the final temperature used is dependent upon the degree of crystallization desired in the product and upon the actual maximum temperature. When the maximum temperature is limited to the range of about 1400.degree. to about 1675.degree. F. it is indicated in that patent application that in the crystalline phase that is formed beta-eucryptite or beta-eucryptite-like crystals predominate. When the final or maximum temperature of the heat treatment is above about 1650.degree. F. the crystals formed constitute a mixture of beta-eucryptite and beta-eucryptite-like crystals and beta-spodumene and beta-spodumene-like crystals. At the maximum heat treatment temperatures of about 1800.degree. to 2100.degree. F. the crystalline phase is primarily beta-spodumene crystals and beta-spodumene-like crystals.
The glass-ceramic of that patent application has an expansion coefficient that is dependent upon the final temperature of the heat treatment. When the glass-ceramic results from a final heat treatment temperature of a maximum of about 1675.degree. F., the expansion coefficient is substantially lower than when a higher final temperature for the heat treatment is used. In other words, the glass-ceramic, in which the crystalline phase can be considered as being beta-eucryptite, has a substantially lower expansion coefficient than the glass-ceramic of the same composition having a crystalline phase that can be considered as being beta-spodumene.
Copending U.S. patent application Ser. No. 464,147 filed by Clarence L. Babcock, Robert A. Busdiecker and Erwin C. Hagedorn on June 15, 1965 and now abandoned, and entitled "Product and Process for Forming Same" with common assignee discloses and claims a further class of thermally crystallizable glass compositions and glass-ceramics made from these glasses. That application is hereby incorporated by reference.
The Babcock et al. patent application Ser. No. 464,147 discloses the discovery that a crystallizable glass composition, containing the following essential components, present in the glass composition in the following weight percent limits, can be treated at a finishing temperature th at can be varied at the maximum within about 50.degree. to 100.degree. F. or more, without affecting the substantially uniform, low expansion characteristics imparted to the transparent glass-ceramic which is formed, the glass-ceramic having a coefficient of linear expansion of about -10 .times. 10.sup.-.sup.7 to about 10 .times. 10.sup.-.sup.7 per .degree. C. over the range 0.degree. to 300.degree. C.
______________________________________ Component Weight Percent ______________________________________ SiO.sub.2 56 - 68 Al.sub.2 O.sub.3 18 - 27 Li.sub.2 O 3.4 - 4.5 CaO 0 - 3 ZnO 0 - 2 B.sub.2 O.sub.3 0 - 4 TiO.sub.2 0 - 6 ZrO.sub.2 0 - 3 MgO 0 - 3 Na.sub.2 O 0 - 1 F.sub.2 O.sub.5 0 - 3 (SiO.sub.2 + Al.sub.2 O.sub.3) at least 82 (SiO.sub.2 +Al.sub.2 O.sub.3 +B.sub.2 O.sub.3 +P.sub.2 O.sub.5) 86 - 91 (CaO+MgO+ZnO+Na.sub.2 O) 2.5 - 6 (SiO.sub.2 +Al.sub.2 O.sub.3 +P.sub.2 O.sub.5+Li.sub.2 O) no more than 93 TiO.sub.2 + ZrO.sub.2 2 - 6 ______________________________________
where the ratio of (CaO+MgO+ZnO+ Na.sub.2 O+B.sub.2 O.sub.3) to Li.sub.2 O is less than 2.4 and the ratio of SiO.sub.2 to Al.sub.2 O.sub.3 is no more than 3.8 and is usually no more than 3.3. An example of this class of thermally crystallizable glasses that provides a glass-ceramic having an average coefficient of linear expansion of 0 .times. 10.sup.-.sup.7 /.degree. C. (0.degree.-300.degree. C.) has the following theoretical composition and for an actual tank batch had the following analyzed composition, expressed as various oxides and one chemical element in weight percent:
______________________________________ Theoretical, % Analyzed, % SiO.sub.2 67.4 67.5 Al.sub.2 O.sub.3 20.9 22.1 CaO 2.7 2.6 ZnO 1.3 0.5 Li.sub.2 O 3.9 3.59 TiO.sub.2 1.8 1.9 ZrO.sub.2 2.0 1.95 Na.sub.2 O 0.5 0.80 Cl 0.2 * Sb.sub.2 O.sub.3 0.4 * ______________________________________ * Not analyzed.
The differences are believed to be due to alumina pick-up and volatilization loss in the case of ZnO.
This glass-ceramic as an article was prepared by placing the glass as an article about 2 inches thick and at 1300.degree.-1700.degree. C. in a preheated oven at 1000.degree. F. The oven temperature was increased to 1150.degree. F. because of the presence of the hotter glass article and oven was then maintained at 1150.degree. F. for 3 hrs. followed by increasing the oven temperature to 1350.degree. F. at a rate of 5.degree. F./minute and then maintained at 1350.degree. F. for 50 hours. The glass article then was cooled at the rate of 1.degree. F./minute until 1000.degree. F. was reached, then 5.degree. F./minute until room temperature was reached.
Copending U.S. patent application Ser. No. 362,481 filed by Robert R. Denman on Apr. 24, 1964and now U.S. Pat. No. 3,428,513, and entitled "Ceramics and Method" with common assignee describes a process of improving the modulus of rupture of certain compositions of glass-ceramic by an ion-exchange process in which lithium ions in a surface layer of the glass-ceramic article are replaced by larger alkali metal ions, specifically sodium or potassium ions. The glass-ceramic is in the form in which the crystalline phase is primarily beta-spodumene crystals and beta-spodumene-like crystals. To date none of the ion-exchange materials used has provided a similar result with a strength increase for a glass-ceramic of such certain compositions, that are specified in said application Ser. No. 464,147 and in the next paragraph in which the crystalline phase is primarily beta-eucryptite or beta-eucryptite-like crystals.
That Denman patent application is also incorporated by reference. The thermally crystallizable glass composition that forms the glass-ceramic used has the following weight percentage limits based on the total composition:
______________________________________ SiO.sub.2 68 - 72 Al.sub.2 O.sub.3 16 - 18 Li.sub.2 O 3 - 4 MgO 3 - 5 ZrO.sub.2 1 - 2 TiO.sub.2 1.2 - 2.4 P.sub.2 O.sub.5 0.8 - 2 ______________________________________
In addition, small amounts of residual arsenic and antimony oxides are often present in the composition, since arsenic or antimony compounds are often used as fining or oxidizing agents. In actual practice, arsenic, expressed as As.sub.2 O.sub.3, is usually present in amounts not more than 0.3 weight percent, and antimony, expressed as Sb.sub.2 O.sub.3, is seldom present in amounts over 1 weight percent. Sodium oxide is often present in the glass to a certain degree from the raw materials, usually in amounts not over 1.5 or 2 weight percent. Further, when As.sub.2 O.sub.3 is used as a fining agent, it is commonly added together with a little NaNO.sub.3, a well-known practice. Another additive sometimes employed is F, usually in amounts not exceeding 0.3 weight percent. It is added as a salt in the usual manner and seems to aid the crystallization process somewhat when it is employed. Thus, it seems to accelerate the rate of crystallization, sometimes to such an extent that harmful exothermic effects result; hence, it is usually undesirable to have any more than 0.25 to 0.3 fluorine present, expressed as weight percent F, in the final glass composition.
This glass is formed, e.g., by melting a batch of suitable ingredients in a gas-fired furnace at a temperature of about 2900.degree. F. and after fining is cooled to a suitable temperature for flowing, casting or any other feeding step to form glass articles which are then heat treated, first, at a low temperature to form many nuclei or crystallites, and thereafter at a higher temperature to complete crystallization to the desired degree. The final maximum crystallization temperature is about 1800.degree. to 2100.degree. F. and the average coefficient of linear expansion (0.degree.-300.degree. C.) is less than 20 .times. 10.sup.-.sup.7 /.degree. C. and usually about 15 .times. 10.sup.-.sup.7 /.degree. C. In one example a glass-ceramic article having, and made from a thermally crystallizable glass having, the following analyzed composition was immersed in a molten bath of sodium nitrate at 750.degree. F. for 1/2 hour for some articles and for 3 hours for others, followed by cooling, water washing and drying:
______________________________________ Ingredient Weight Percent ______________________________________ SiO.sub.2 70.4 Al.sub.2 O.sub.3 16.8 MgO 4 Li.sub.2 O.sub.3 3.5 ZrO.sub.2 1.3 TiO.sub.2 1.8 P.sub.2 O.sub.5 1.5 F 0.09 Na.sub.2 O 0.5 As.sub.2 O.sub.3 0.15 ______________________________________
The original glass had an annealing point of about 1220.degree. F. The ion-exchanged glass-ceramic articles had very high unabraded and high abraded values of modulus of rupture.
The preceding paragraphs refer to ion exchange for the purpose of improving the strength of a specific type of glass-ceramic. Before that invention the prior art had disclosed the technique of changing the properties of glass articles by ion exchanging one alkali metal for another in the surface layer of the glass article. This ion-exchange process can be one of two types of substitution. In the one embodiment the replacing alkali metal ion has a larger ionic diameter or radius than the alkali metal ion being replaced. In the second embodiment the replacing alkali metal ion has a smaller ionic diameter than that of the alkali metal ion being replaced. H. G. Fischer and A. W. LaDue disclose and claim in their copending U.S. patent application Ser. No. 504,159, filed on Oct. 23, 1965 now U.S. Pat. No. 3,481,726, and entitled "Process and Product" with common assignee, a method in which ion exchange of one alkali metal for another is accomplished by using a liquid medium containing an alkali metal salt or an organic acid.
E. F. Grubb and A. W. LaDue in their copending U.S. patent application Ser. No. 529,215, filed on Feb. 23, 1966 now U.S. Pat. No. 3,498,773, and entitled "Process and Product" with common assignee, discloses and claims another ion-exchange method in which the alkali metal ion, that is to substitute for another alkali metal ion in the surface layer of the glass article, is used as a compound that is not molten when in contact with the glass at the elevated temperature used for the ion exchange.
In view of the m ethods of said Fischer et al. and Grubb et al., hereby incorporated by reference, it will be apparent that there have not been developed several different techniques for ion exchange.
The ion-exchange process has been used to treat glass, that is not thermally crystallizable, to convert a surface portion of the glass article to a composition that at the temperature used for the ion exchange, if sufficiently high, will crystallize to form a glass-ceramic in which the crystals are referred to as beta-spodumene crystals. If the entire article were of this surface layer composition, it is reasonable to expect for some specific compositions that only the surface would crystallize. The product of this process is referred to as a surface crystallized glass article, as distinguished from the conventional glass-ceramic which is commonly referred to as bulk crystallized product. As a result of this process in which sodium ions are replaced by lithium ions of a molten lithium salt bath in which the glass article is immersed at the elevated temperature, the compositonal change in such that surface crystallization occurs while the main body or interior portion remains unchanged in composition and thus remains as glass. This process is disclosed in U.S. Pat. No. 2,779,136.
Copending U.S. patent application Ser. No. 371,089, filed on May 28, 1964 now U.S. Pat. No. 3,528,828, by William E. Smith and entitled "Glass, Ceramics and Method" with common assignee, discloses and claims a type of thermally crystallizable glass that, for example, as a glass has an expansion coefficient of about 90 .times. 10.sup..sup.-7 /.degree. C. but as a glass-ceramic has an expansion coefficient between 100 .times. 10.sup..sup.-7 /.degree. C. and 120 .times. 10.sup..sup.-7 /.degree. C. For the glass-ceramic the actual value of the coefficient is determined by the temperature and time of heat treatment for the controlled crystallization. The crystalline phase of that glass-ceramic is nepheline. That application is hereby incorporated by reference.
Another type of composition of thermally crystallizable glass and the glass-ceramic made from it are disclosed in British Pat. Specification No. 869,328. The ingredients include silica, alumina and soda and thus the glass-ceramic has nepheline as its primary crystalline phase.
William E. Smith in another copending U.S. patent application, which is application Ser. No. 532,058, filed on Mar. 7, 1966 now U.S. Pat. No. 3,486,963, and entitled "Process and Product" with common assignee, discloses and claims a process for treating an article of a glass-ceramic to convert at least an area of its surface layer to a noncrystalline glass under conditions to maintain the main body portion of the article as a glass-ceramic. That invention requires that the glass-ceramic has a coefficient of linear expansion that it at least 5% and is a maximum of about 200% greater than that of the noncrystalline glass of the same overall chemical composition, which, of course, is the glass formed as a surface layer by the process. That application is hereby incorporated by reference.